There are two types of images that you can upload for each of your events.
This is the landscape image that is used on the Event Description page
You can choose an aspect ratio of 5:2, 4:1, or 2:1 therefore any image size with these ratios will work. You can actually upload any image you like and ECA will try and crop it for you if the dimensions are not perfect. Here are some suggested sizes:
5:2 - At least 1000px x 400px
4:1 - At least 1000px x 250px
2:1 - At lease 1000px x 500px
The thumbnail uses a 1:1 aspect ratio - think perfectly square like Instagram.
I recommend using an image that is at least 600px x 600px, especially if you want to use tile view.
What if my images are looking low quality or blurry?
This usually means that the image isn't high quality enough. For example, if your thumbnail is 100px x 100px but your widget is full screen then people on large monitors will see a very low quality image.